On Fri, 2008-08-29 at 14:03 -0400, Gregory Haskins wrote:> *Patch submitted for inclusion in PREEMPT_RT 26-rt4. Applies to 2.6.26.3-rt3*> > Hi Ingo, Steven, Thomas,> Please consider for -rt4. This fixes a nasty deadlock on my systems under> heavy load.> > [> Changelog:> v2: only touch seqlock_t because raw_seqlock_t doesn't require> serialization and userspace cannot modify data during a read> > v1: initial release> ]> > -Greg> > ----> seqlock: serialize against writers> > Seqlocks have always advertised that readers do not "block", but this was> never really true. Readers have always logically blocked at the head of> the critical section under contention with writers, regardless of whether> they were allowed to run code or not.> > Recent changes in this space (88a411c07b6fedcfc97b8dc51ae18540bd2beda0)> have turned this into a more explicit blocking operation in mainline.> However, this change highlights a short-coming in -rt because the> normal seqlock_ts are preemptible. This means that we can potentially> deadlock should a reader spin waiting for a write critical-section to end> while the writer is preempted.

Ah, the point I was missing is higher-priority realtime task, in whichcase the write side will never run because it wont preempt.

> This patch changes the internal implementation to use a rwlock and forces> the readers to serialize with the writers under contention. This will> have the advantage that -rt seqlocks_t will sleep the reader if deadlock> were imminent, and it will pi-boost the writer to prevent inversion.> > This fixes a deadlock discovered under testing where all high prioritiy> readers were hogging the cpus and preventing a writer from releasing the> lock.> > Since seqlocks are designed to be used as rarely-write locks, this should> not affect the performance in the fast-path

Still dont like this patch, once you have a rwlock you might as well goall the way. Esp since this half-arsed construct defeats PI in certaincases.